I) Field of the Invention
This invention relates to the field of lignocellulosic material production, in particular, to the bleaching and brightness stabilization of lignocellulosic materials.
II) Brief Description of the Prior Art
Lignocellulosic materials such as wood are the raw materials used for the production of pulps and papers. In order to make papers, lignocellulosic materials are first reduced to pulps of discrete fibres by a mechanical or chemical pulping process. In mechanical pulping, pulps are produced, with retention of lignin, mainly through the action of mechanical forces in a yield of 90-98%. One example of a mechanical pulp is the so-called thermomechanical pulp (TMP) produced from the thermomechanical pulping process. In chemical pulping, pulps are produced in a yield of 45-55% through the dissolution of most of the lignin by the pulping chemicals at an elevated temperature. The most dominant chemical pulp in use today is the so-called kraft pulp produced from the kraft pulping process where sodium hydroxide and sodium sulfide are used as the pulping chemicals.
Mechanical and chemical pulps typically have a pale-yellow and a deep brown colour, respectively. Bleaching of these pulps to a whiter colour is often needed prior to the process of papermaking. The whiteness of pulps and papers is commonly estimated by the ISO (International Standardization Organization) brightness determination, which measures the directional reflectance of light at 457 nm of the papers in an Elrepho instrument [TAPPI Test Methods, T 452 om-92, Tappi Press: Atlanta, 1996]. A low brightness such as 30% ISO indicates deep brown papers and a high brightness such as 85% ISO represents white papers. Unbleached mechanical wood pulps and chemical (kraft) pulps typically have ISO brightness values of 45-65% and 30-40%, respectively, depending on the wood species and the pulping conditions.
The current industrial processes for the bleaching of mechanical pulps are the alkaline hydrogen peroxide process and the sodium dithionite (hydrosulfite) process [Dence and Reeve, Pulp Bleaching-Principles and Practice, Tappi Press: Atlanta, p.457-512, 1996]. Alkaline hydrogen peroxide, in the presence of peroxide stabilizers such as sodium silicate and magnesium sulfate, is capable of bleaching mechanical pulps such as spruce TMP from an initial brightness of 55-60% to 70-80% ISO. However, alkaline peroxide bleaching, being an oxidative process, reduces the yield of the pulps by 2-5% and produces effluents with high chemical oxygen demand (COD) [Soteland et al., 1988 International Pulp Bleaching Conference Proceedings, Tappi Press: Altanta, p.231, 1988]. Sodium dithionite bleaching is a reductive and more selective process. However, it is less effective than alkaline hydrogen peroxide bleaching in terms of maximum brightness gain. The process normally needs to be carried out at a lower consistency to reduce the amount of air entrained in the pulps to minimize the oxidation of sodium dithionite during bleaching [Dence and Reeve, Pulp Bleaching-Principles and Practice, Tappi Press: Atlanta, p.500, 1996]. Consistency is the weight percentage of pulp in a pulp and water mixture; bleaching at a lower consistency requires the use of more water and is less desirable. In addition, some of the dithionite undergoes disproportionation during bleaching to give sodium bisulfite and sodium thiosulfate that is corrosive to paper machines [Garner, J. Pulp Paper Sci. 14(5): J51-57, 1984]. Both peroxide-bleached and dithionite-bleached pulps are highly unstable; they rapidly turn yellow with loss of the brightness gained from bleaching when exposed to light and/or heat or during storage [Leary, J. Pulp Paper Sci. 20(6): J154-160, 1994].
Partial or full bleaching of kraft pulps is currently accomplished with various oxidative bleaching chemicals such as oxygen, chlorine dioxide and ozone, and alkali extraction in several stages [Dence and Reeve, Pulp Bleaching-Principle and Practice, Tappi Press: Atlanta, p.213-361, 1996]. One problem with oxidative bleaching is a loss of pulp yield because of the low bleaching selectivity.
Alternative chemicals for the bleaching of lignocellulosic materials, particularly mechanical wood pulps, have been reported sporadically over the past twenty years or so. Bleaching of thermomechanical pulps has been achieved with thiol compounds [Kutney, J. Pulp Paper Sci. 12(4): J129-131, 1986], amino boranes [Pedneault, et al., Pulp Paper Can. 98(3): 51-54, 1997], and a spirophosphorane or a hypophosphorous acid [Djerdjouri and Robert, Proceedings of 9th International Symposium on Wood and Pulping Chemistry, 23-1-23-3, 1997]. Unfortunately, a very high dosage of these chemicals is needed to give a limited brightness gain. For example, 3.0% (on OD pulp) of ethanedithiol is needed to give a brightness gain of 6.0 ISO points. In addition, thiol compounds are too toxic and malodorous, and amino boranes too expensive to be used commercially.
Tris(hydroxymethyl)phosphine (THP), P(CH2OH)3, a water-soluble tertiary phosphine, has been used for the synthesis of water-soluble organometallic complexes [Ellis et al., Inorg. Chem. 31: 3026-3033, 1992; Higham, et al., Chem. Commun. 1107-1108, 1998]. Some of these complexes have also been used as catalysts for the catalytic hydrogenation of cinnamaldehyde and hydroformylation of pent-1-ene [Fujuoka et al., Chem. Commun. 489-490, 1999]. Quaternary phosphonium compounds such as tetrakis(hydroxymethyl)phosphonium chloride (THPC), [P(CH2OH)4]Cl and tetrakis(hydroxymethyl)phosphonium sulfate (THPS), [P(CH2OH)4]2SO4 have been used as basic chemicals to make commercial flame(fire)-retardants for textiles [Calamari and Harper, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed. Vol. 10, 998-1022, 2000]. THPS has also been shown to be a non-hazardous biocide for the control of hydrogen sulfide emissions and the reduction of corrosion in paper mills [Haack et al., 1997 Tappi Engineering & Papermakers Conference Proceedings, Tappi Press: Atlanta, 1115-1119, 1997]. The ability of THP and THPS to kill catalase-producing bacteria in pulping liquors used for hydrogen peroxide bleaching of wood pulps has also been reported [Bowdery et al., PCT WO 01/53602 A1, 2001]. Water-sensitive, trimethyl phosphite, P(OCH3)3, has been reacted with mechanical wood pulps in anhydrous dichloromethane to allow the determination of o-quinones in the pulps by 31P NMR [Lebo et al., J. Pulp Paper Sci. 16(5): J139-143, 1990; Argyropoulos et al., Holzforschung 46(3: 211-218, 1992]. When coated onto the surface of papers made from mechanical pulps, sodium hypophosphite, H2P(O)ONa [Violet et al., Cellul. Chem. Technol. 24: 225-235, 1990] and sodium hydroxymethylphosphinate, HOCH2P(O)(H)ONa [Guo and Gray, J. Pulp Paper Sci. 22(2): J64-70, 1996] have been shown to improve the brightness stability of papers.
U.S. Pat. No. 5,580,422 issued to Hoechst Celanese Corporation on Dec. 3, 1996 describes the brightening of color dyed wastepaper with a bleaching agent in the presence of a quaternary compound based on “nitrogen and phosphorous”. All the quaternary compounds described contain at least one long-chain (C14-C22) alkyl or alkenyl group, or preferably one straight-chain hexadecyl (C16) group. In addition, a known bleaching agent such as sodium hydrosulfite or hydrogen peroxide is required for the bleaching which is limited to pulp from color dyed wastepaper.
Prior to the present invention, however, no water-soluble phosphines or phosphonium compounds including THP, THPC and THPS have been used alone for the bleaching or brightness stabilization of lignocellulosic materials such as wood pulps and papers.